1. Field of the Invention
This invention relates to an improved single-stage input-current-shaping flyback converter with fast output-voltage regulation, and, more particularly, to a single-stage, single-switch input-current-shaping flyback converter with reduced conduction losses in the primary side of the converter.
2. Discussion of the Related Art
In single-stage input-current-shaping (S2ICS) converters, input-current shaping (ICS), isolation, and high-bandwidth output-voltage control are performed in a single conversion step (i.e., without creating a regulated DC bus, as is commonly found in two-stage ICS converters). Most of the S2ICS circuits integrate a boost ICS stage with a forward or flyback DC/DC-converter stage. Generally, S2ICS converters meet European and/or Japanese regulatory requirements regarding the line-current harmonic limits, but they do not improve the power factor (PF) and reduce the total harmonic distortion (THD) as much as their two-stage counterparts. Typically, PF for the S2ICS converters is between 0.8 and 0.9 with a THD in the 40-75% range.
An input-current-shaping circuit of a simple, cost-effective, and efficient S2ICS flyback converter is described in U.S. Pat. No. 5,991,172 (the “Jovanovic '172 Patent”) to M. M. Jovanovic and L. Huber, entitled “AC/DC flyback converter with improved power factor and reduced switching loss,” and U.S. Pat. No. 6,005,780 (the “Hua '780 Patent”) to G. Hua, entitled “Single-stage ac/dc conversion with PFC-tapped transformers”. FIG. 1 shows such an input-current shaping circuit in S2ICS converter 100. In FIG. 1, ICS in S2ICS converter 100 is achieved by adding inductor 101 (LICS) in series with rectifier 102 (DICS) to a conventional AC/DC flyback converter without PFC. Inductor 101 and rectifier 102 are connected between the positive terminal of full-bridge rectifier 103 (FBR) and primary-winding tap 105 of flyback transformer 104 (T).
By connecting inductor 101 and rectifier 102 in series to primary-winding tap 105 of flyback transformer 104, the voltage across energy-storage (bulk) capacitor 106 (CB) can be limited to a desired level (e.g., 400 V at the universal line range of 90-264 Vrms) In fact, when switch 108 (SW) is closed (“on”), winding 107a (N1), which is a portion of primary winding 107 (NP) of flyback transformer 104, appears in series with ICS inductor 101. When switch 108 is open (“off”), winding 107b (N2), which is the other portion of primary winding 107 (NP), appears in series with ICS inductor 101. Thus, regardless of whether switch 108 is open or closed, the voltage across the portion of primary winding 107 conducting the ICS inductor current opposes the rectified line voltage Vin across capacitor 109 (Cin). Consequently, the volt-second balance of the ICS-inductor core is achieved at a substantially reduced bulk-voltage level. In addition, winding 107b provides a direct energy transfer path to the output load 110 when switch 108 is off, thereby improving conversion efficiency. Performance in S2ICS converter 100 is optimized by varying the tap location on the primary winding 107 of flyback transformer 104.
Input-current shaping inductor 101 is usually designed to operate in discontinuous conduction mode (DCM). Under DCM operation, low input-current harmonic distortion is achieved because a DCM boost converter inherently draws a near sinusoidal current if its duty cycle is held relatively constant during a half line cycle. If the inductance of ICS inductor 101 exceeds its maximum value for DCM operation, inductor 101 operates in the continuous conduction mode (CCM) during a narrow interval near the peak of the rectified line voltage. Generally, a larger inductance of inductor 101 increases converter efficiency and decreases input-current ripples. However, a larger inductance of inductor 101 also decreases input power factor and increases line current harmonics.
A flyback transformer can operate in DCM, CCM, or at the DCM/CCM boundary. As described in the Jovanovic '172 patent, turn-on switching loss can be substantially reduced by operating flyback transformer 104 at the DCM/CCM boundary. To operate flyback transformer at the DCM/CCM boundary over the entire line and load range, a variable switching-frequency control circuit is used. Control circuit 120 provides the required control signal SW for periodically opening and closing switch 108.
Input-filter capacitor 109 can be connected on either the DC side of full-wave bridge rectifier 103 (such as shown in FIG. 1), or the AC side of full-wave bridge rectifier 103. Similarly, input-filter inductors 111a (Lin1) and 111b (Lin2) can be connected on either side of full-bridge rectifier 103.
With ICS-inductor 101 operating in DCM, S2ICS flyback converter 100 is well suited for universal-line applications (e.g., in a notebook adapter or charger). In such a configuration, the line current quality of S2ICS converter 100 is approximately the same at low-line and high-line voltages, as explained in U.S. Pat. No. 5,757,626 (the “Jovanovic '626 Patent”) to M. M. Jovanovic and L. Huber, entitled “Single-stage, single-switch, isolated power-supply technique with input-current shaping and fast output-voltage regulation”. However, the DCM operation of the ICS inductor results in a larger current stress on switch 108 and larger input-current ripple. A larger current stress on switch 108 reduces converter efficiency, and a larger input-current ripple requires a larger input filter. Furthermore, the conduction loss of the primary-side rectifiers (i.e., full bridge rectifier 103 and rectifier 102) in FIG. 1 is high because the current of ICS inductor 102 always includes three rectifiers: rectifiers 103a, 102, and 103d conduct during a positive half cycle of line voltage vin, and rectifiers 103b, 102, and 103c conduct during a negative half cycle of line voltage vin. Generally, S2ICS flyback converter 100 in FIG. 1 is limited to operation at power levels below 100 W.
To improve efficiency and power level, two S2ICS flyback converters can be interleaved—i.e., two converters can be connected in parallel, with the switching instances of the primary gate signals phase-shifted by 180°. With interleaving, input and output filter sizes can be significantly reduced and the total power loss can be evenly distributed between the two parallel converters. However, interleaving of two converters significantly increases the number of components. Furthermore, if the flyback transformer operates at the DCM/CCM boundary, interleaving of variable-frequency converters requires a relatively complex control circuit.
It is thus desired to improve efficiency and to increase the maximum power level of the S2ICS flyback converter by reducing primary-side conduction losses caused by increased input-current ripple and increased rectifier conduction losses in a simple and cost-effective way.